In a small saucepan, bring vinegar, sugar, 1 teaspoon salt, 1/2 teaspoon pepper, and 1 cup water to a boil over medium-high. Reduce to a simmer and cook until sugar is dissolved, about 2 minutes. Remove from heat and add cucumbers. Cover and let cool until cucumbers are softened, 10 minutes.
Assemble sandwiches with bread, mustard, pastrami, Swiss, pickles, and mayonnaise.